The present invention generally relates to semiconductor fabrication, and more particularly to the use of electrolytic plating to control solder wettability of a copper pillar.
Flip chip technology has been widely used as it allows a high I/O count, high density interconnection scheme with proven performance and reliability. Solder bumps are deposited on contact pads on chip surfaces and the chips are then flipped and positioned such that the solder bumps are aligned with matching pads of an external circuit. Solder reflow completes the interconnection process, after which underfill material is introduced to fill the spaces about the interconnections.
Flip chip interconnection assemblies have included copper pillars having solder caps. The copper pillars and solder caps may each be formed by electroplating. The advantages in copper lie in the extendability to finer pitch and the superior electromigration (EM) performance compared to conventional solder C4's. The copper pillar provides the enhanced EM performance but increases the die stress for chip-to-chip and chip-to-wafer joints (i.e., 3D applications). The finer pitch is due to its vertical sidewall.
In copper pillar technology, a small amount of solder is still required to connect the copper pillars on the chips to the pad on the substrate. However, copper pillar-based solder connectors are relatively expensive to fabricate and also difficult to prevent the solder from wetting to the copper pillar sidewall. Such wetting to the sidewall will reduce the standoff, therefore limiting underfill flow and can sometimes lead to solder bridging.
Copper pillars are used to increase the reliability of the packaged chip because it is a taller structure that joins two entities (silicon chip and organic package, for example) which may be severely mismatched in terms of their coefficients of thermal expansions (CTEs). It is true that the additional height of pillars in comparison to standard solder bump interconnects produce increased strain for a given amount of expansion mismatch for a single interconnect of the same diameter. However, a copper pillar with non-wettable sidewalls allows either more interconnects for a given area or a larger diameter which help to offset or even reduce the increase in strain. The increase in height provides improvement for both cleaning after chip attach and underfill filling of the module. Both of these improve reliability of the module.